Combining results from human tumors and a mouse model, researchers publishing in Nature Communications have shown that senescent cells promote the growth of glioblastoma, a form of brain cancer.
Deadly, aggressive, and hard to treat
Glioblastoma is one of the most dangerous forms of cancer. Even surgery, radiotherapy, and chemotherapy do little against this invasive disease, and patients diagnosed with it survive for less than 15 months on average . Glioblastoma suppresses the immune system’s efforts to remove it, and this paper shows how this seems to relate to the senescence-associated secretory phenotype (SASP).
Senescent cells have a double-edged relationship with cancer. While senescent cells normally do not divide, and cancer cells divide uncontrollably, there are more complicated biological effects in play. The SASP can promote immune clearance in some cases , but in other cases, it can directly promote tumor growth  and suppress the immune system in a way that encourages tumor growth . One particular aspect of the SASP, interleukin 6 (IL-6), has been shown to promote cancer proliferation , and driving some cancers to senescence has shown short-term benefits but long-term harm, as the SASP drives their growth .
There are four distinct cell subpopulations in malignant glioblastoma tumors, relating to their phenotypic closeness to different types of brain cells, and the proportions of these subpopulations determine which of three general types it sits in . Specifically, mesenchymal tumors, which contain cells similar to mesenchymal stem cells, are anti-inflammatory in a harmful way: they promote tumor-associated macrophages, which discourage immune clearance, and they are harder to treat with radiation therapies .
Senescent cells encourage deadlier tumors
This study began with tumors taken from human beings. Staining for the common senescence marker SA-β-gal confirmed that senescent cells were present in these tumors, and additional analysis of p53 mutations found that these cells were themselves cancerous. There was no evidence that any specific molecular change was associated with the proportion of senescent cells.
The researchers then employed a mouse model of glioblastoma that is intended to accurately recapitulate the human condition. These mice had tumors that were very like their mesenchymal human counterparts, including in gene expression related to cellular senescence. There were senescent cells throughout these tumors, in both dying and proliferating regions, which comprised roughly 2% of the overall tumor size.
These mice were also modified to have senescent cells that die easily upon injection of a specific drug. Injecting this drug, which functioned as a senolytic in the mice, significantly improved their survival, as did injecting them with the well-known senolytic ABT-263.
As expected, genes related to the SASP were downregulated in the tumors of the treated mice. Critically, the gene expression of these tumors was also shown to shift away from the mesenchymal type and towards the less harmful types. The effects on genes related to tumor-associated macrophages were similarly beneficial.
NRF2, a common transcription factor that has antioxidant properties and is usually associated with preventing tumor growth, was found to promote tumor growth in this case. Knocking out NRF2 had similar effects as the senolytic treatments.
These results come from cells and mice, so they don’t prove that senolytics will be effective against glioblastoma in human beings. However, the amount of supporting data, and the identification of the specific molecular causes and gene expression profiles in the presence of the SASP, makes it clear that targeting senescent cells is worth exploring as a strategy to be used alongside existing therapies against glioblastoma.
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